Upton Anniversary Clock in Meccano

British Horological Institute celebrates 150 years

A special clock was designed and built by members of the BHI to celebrate 150
years

A three pendulum clock with almost free escapement was designed by Jim
Arnfield and built by a small team and is on display at the BHI at Upton Hall,
Newark.

The escapement drives three pendulums. It rotates to drive each
pendulum in turn and so is three dimensional.

The pendulums are on knife edges. Their supports are arranged in a triangle
in a platform which has a central recess in which the escapement rotates. Each
pendulum has an impulse lever at its upper end pointing inwards at right angles
to its knife edges. Each impulse lever is visited in turn by the impulse arm of
the escapement.

The escapement is mounted in a vertical central shaft which bears two arms
which rotate with the shaft. The upper is the locking arm. It is caught at its
tip by a stop (pointing downwards) underneath the pendulum support platform,
positioned so that it (and the impulse lever below in line with it) come to a
stop opposite a pendulum impulse lever.

Just below the locking arm is the impulse arm. There is a small roller at one
end which rolls against the pendulum lever. As the impulse and locking arms
rotate into impulse position, it is reset just in time. It thus comes to lie
just above the tip of the pendulum impulse arm. Its other end has a small lever
which catches on a reset arm mounted on the pendulum support platform between
the escapement stops. This catch is the heart and soul of the escapement. As the
impulse arm catches it as it is flying around between impulses, the reset arm is
depressed, but at the same time the impulse arm at this end is also depressed
which means that its impulse or business end is raised - to a higher level ready
for impulse and locked there. I asked Jim Arnfield how on earth he had conceived
of this arrangement and his answer was that he seems to have a gift for
imagining mechanisms.

As the pendulum is just completing its swing before impulse, its impulse arm
tip catches under the impulse roller and lifts the impulse arm slightly. As a
result its other end is lowered - and just enough to clear the reset arm. The
reset arm of course springs back (to its stop) to its previous position ready
for the next operation.

The impulse arm is now ready to give impulse, and does so and its roller then
falls off the end of the pendulum impulse arm. It continues to fall further and
its other end then raises the end of the locking arm to cause unlocking. The
cycle is then ready to repeat.

Meccano Upton BHI 150 Anniversary Clock showing three pendulums

Detail of the escapement

The clock is weight driven and the weight is a heavy circular ring outside
the pendulums, suspended from three cords each attached to one circular drum
supported on rollers. This drum is linked to the escape shaft via a
differential through which the electrical rewind also operates.